In the automatic operating door industry, as well as in other industrial fields, electric motors are used to operate doors and other components. The electric motors can usually be started in either direction of rotation (i.e., clockwise or counterclockwise), and they are allowed to run for a certain period of time after which they are completely stopped. For example, a motor will be energized to turn in the clockwise direction to open a door until a limit switch in the control circuit is operated by the opening door, whereupon the motor is deenergized and stopped. Subsequently, the motor will be energized to turn in the counterclockwise direction to close the door until a further limit switch in the control circuit is operated by the closing door, whereupon the motor is deenergized and stopped. Any standard general purpose motor can be used with no difficulty.
In some applications, such as motor driven doors, there is a need for instantly stopping the motor and reversing its direction. For example, motor driven doors when they are closing can damage persons or equipment if they are not provided with some sort of sensing equipment (e.g., pneumatic or mechanical safety edges, sound beams, or photoelectric beams) which will detect an obstruction in the pathway of the door and be used to cause the motor to stop instantly and immediately start in the opposite direction so as to open the door. Such safety equipment is intended to minimize the risk of damage to the door equipment and to the obstruction in the pathway of the door. In particular, such safety equipment is required if the door is likely to strike a person or a pet animal.
Instant reversal is readily available in the case of three phase motors simply because of the manner in which such motors are started and operated. The instant reversal safety feature can be used with standard three phase motors without any special connections or expensive control circuits. However, in the case of single phase motors, an instant reversal of the motor is not possible unless a special purpose motor and a special purpose control circuit with expensive direction sensing switches and current operated reversing relays is used. Although such special purpose motors are conventional and are available on the open market, they are considerably more expensive and in lower supply than a general purpose motor of the same Horsepower rating. Such general purpose motors are usually of the squirrel cage design and use a capacitive start winding temporarily connected in the circuit to provide a phase shift needed to start the motor. This type of capacitor start single phase squirrel cage electric motor is conventional and has been commercially available for a long time.
Thus, there is need for an inexpensive control circuit for use with a conventional single phase, squirrel cage electric motor that is easily and inexpensively adaptable to such motors.